The present invention relates to an apparatus for measuring the thickness of films such as plastic films,. wherein such measurements are made by detecting capacitance variations when the film is placed in proximity to a sensing head, wherein variations in film thickness cause corresponding dielectric variations which are detected as variations in measured capacitance.
The use of capacitance measuring sensors for measuring film thickness is known in the art, one form of which is disclosed in U.S. Pat. No. 3,764,899, issued Oct. 9, 1973, and owned by the assignee of the present invention. This patent measures film thickness by passing the film through a capacitance sensor, wherein variations in film thickness are sensed as dielectric thickness variations between the capacitance members, and are detected as variations in a signal which is applied to the capacitance members.
Another form of capacitance measuring device is disclosed in British Patent No. 2,038,483, issued July 20, 1983, and owned by the assignee of the present invention. This patent discloses a capacitance sensor for sensing film thickness changes wherein the film is passed over capacitance elements arranged in side-by-side relationship, variations in film thickness being detected as variations in a signal applied to an electrode positioned in an air gap over which the film is moved. The present invention is an improvement in the construction of the capacitance sensor disclosed in the foregoing British patent.
Measurement instruments of the type for which the invention finds utility are typically used to measure plastic film thicknesses in the range of 0.001-0.050 inch (1-50 mil). Such instruments should have a sensitivity to enable them to detect variations in film thickness of plus or minus 0.1 percent, and subsequent measurements should be repeatable to within about 1 percent. Thus, when a 1 mil plastic film is measured its thickness should be determined with accuracy of plus or minus 0.001 mil, in order to provide results which may be useful in the industrial process control of manufacturing the plastic film. Measurement accuracy should be maintained over a range of ambient temperatures which are found in the typical environment where such instruments are used. In industrial process control applications it is not unusual for ambient temperature changes of plus or minus 10.degree. C. to occur.
A problem in prior art measurement devices has been the inability to maintain accurate measurements of film thickness over a range of ambient temperature changes, due to the fact that the measuring device itself undergoes temperature changes which cause measurement changes exceeding the permissible accuracy range. For example, it has been found that a prior art measuring device introduced measurement errors of a magnitude approximately equal to 1 mil of equivalent plastic film thickness. Thus, if the device were used to measure plastic film of 1 mil thickness, the measurement errors caused by the temperature effects on the measurement device itself could generate a thickness measurement error of approximately 100 percent. An examination of the prior art device has led to the conclusion that the effects of temperature on the measurement device are caused both by temperature expansion and contraction of the measurement components themselves, and also by the temperature coefficients of the dielectric materials associated with the measurement instrument. Accordingly, it is an object of the present invention to provide a measurement device having a sensor head which provides exceedingly low measurement errors resulting from temperature effects on material expansion and dielectric changes.
It is another object of the present invention to provide a capacitance sensor head which is operable to within less than 0.1 percent sensitivity over ambient temperature changes of plus or minus 10.degree. C.